"There is always a risk of being bogged down but in Moscow, we are talking about an operation of three to four months," Alexei Pushkov, an ally of Russian President Vladimir Putin, told French radio station Europe 1 on Friday, according to Reuters.

He added that the strikes were going to intensify.

Pushkov said the strikes mainly targeted Islamic State forces in spite of reports they had concentrated on opponents to Syrian President Bashar Al-Assad.

"The opponents to Bashar are very close to Daesh (Islamic State)," Pushkov said. U.S. sources have said the Russians actually hit facilities of a U.S.-backed group, some of whose rebels received training and support from the CIA.

Pushkov said the U.S.-led coalition had "pretended" to bomb Islamic State forces for a year.

Pushkov said there were would be first contacts between U.S. and Russian military officials on Friday to discuss operations in Syria, where the Cold War superpowers are engaged in combat over the same country for the first time since World War Two.

On Friday, Pushkov also said Russian submarines with nuclear missile launching capabilities patrolling the Atlantic were a "response" to NATO's plans to build up its operations in Eastern Europe. "These are the rules of the game.... So this is a response," Pushkov said.